Combined cycle power generating systems use a medium, e.g. water, whose boiling point and heat capacity are appropriate to the system's operating temperature. This medium is generally heated in a separate heat source such as a steam generator by means of concentrated solar radiation, combustion of fossil fuel, etc.
In combined cycle plants, the combustion of gaseous fossil fuels directly in a turbine is used to turn a gas turbine. The thermal energy liberated during combustion is then used as a heat source to generate steam which, as mentioned above, can drive a steam turbine. This combined gas and steam turbine cycle provides a very high degree of efficiency compared to conventional steam turbines.
In order to maximize efficiency, very exacting requirements are place on the materials used in the turbine, particularly in respect of their temperature and corrosion resistance. The gases in the turbines are very hot, very aggressive and possess high corrosion potential. Special alloys and steels therefore have to be used for the turbines or more specifically their blades in order to reduce e.g. stress crack corrosion. Another possibility for reducing stress crack corrosion and therefore increasing the service life of the turbines is to select a suitable coating for the turbine blade. Chromium-aluminum-yttrium alloys with cobalt, nickel or iron (MCrAlY alloy) or chromium carbide (Cr2C3) combined with chromium and nickel have been found to be advantageous.
It is important that these coating materials exhibit minimum ductility and very high toughness, particularly on the rotating turbine blades, in order to prevent peeling of the turbine blade coating. If the coating were to peel off, the underlying material of the turbine blade would be more or less unprotected from the aggressive gases. The turbine blade material would therefore be prone to stress crack corrosion damage. In addition, surface unevennesses would no longer ensure an optimum flow characteristic, accompanied by an undesirable loss of efficiency.
During combined cycle operation, very high temperatures occur, ranging from 800 to 1500° C. in gas turbines and from 400 to 600° C. in steam turbines depending on the system. These temperatures essentially determine the efficiency of the turbine. Even a deviation of few degrees can mean a significantly lower efficiency in the order of a few percent. However, the temperature is not only critical for the efficiency, but also determines the corrosion potential of the gases and dictates the ductility and toughness of the coating. The material properties of the coating used are therefore mainly optimized for a particular predefined temperature range. In this temperature range, reliable operation of the turbine can be guaranteed without damage to the turbine materials and in particular the turbine blade coatings. The exceeding, particularly for a long period, of a temperature limit can permanently damage the turbine. It is imperative to prevent this. It is therefore important to know whether the turbine has been operated above a specified temperature.
To this end, a plurality of methods for temperature measurement in turbines are known.
For example, various methods for measuring temperature are described in DE 197 36 276. The underlying physical measuring principles are based on, among other things, the temperature dependence of an electrical resistance, the Seebeck effect (thermocouple), a color reaction (thermopaint), the temperature-dependent speed of sound in gases or the spectral distribution of scattered or emitted electromagnetic thermal radiation.
Temperature sensors for turbines must be able to withstand extreme operating conditions in respect of temperature, pressure and vibration. Conventional thermocouples age very quickly under these extreme service conditions. In addition, the temperature often also has to be measured on rotating parts, which is mostly only possible using very complex telemetry. The use of thermopaints has hitherto been limited to experimental studies and is therefore not yet sufficiently reliable for the abovementioned operation in turbines. Although active laser measuring methods such as Rayleigh scattering or CARS (coherent anti-Stokes Raman scattering) are contactless, they are technically extremely complex and difficult to implement.
EP 1 645 538 A1 discloses a material composition for producing a coating, wherein the matrix material of the composition possesses in particular glass ceramic basic properties. Nanoparticles with a particle size ≦1 μm are embedded in the matrix material as a filler.
DE 25 34 668 discloses the use of so-called thermographic paints on ceramic bodies, which are used for determining surface temperature conditions e.g. in furnaces. The exceeding of a particular temperature defined by the properties of the thermographic paint can be demonstrated by the thermographic coating changing color.
According to DE 195 37 999 A1, such thermographic paints can also be applied to the surface of gas turbine components subject to severe thermal stress in order to enable the temperature conditions present to be analyzed at the development stage. A color change e.g. during operation can be captured by a digital camera and allows conclusions to be drawn about the stress-dependent temperature profile in the turbine.